To more effectively utilize the frequency bandwidth of telephone lines (e.g., copper wires) for data transmission, ADSL systems have been developed. ADSL utilizes a multi-carrier technique called Discrete Multi-Tone (DMT) for data transmission. DMT separates the available bandwidth into many channels or carriers for transmission of data. Each channel uses Quadrature Amplitude Modulation (QAM) to carry about 1–15 bits/channel. The signals in each channel are modulated before transmission and demodulated on the other end.
The transmission capability of the individual channels is evaluated for each connection. The bits of data to be transmitted in the ADSL system are grouped into symbols. The data is assigned to the available channels, depending on the number of bits each channel or subcarrier can transmit. A frequency domain vector set is created by encoding the channels. The frequency domain vectors are modulated and converted to digital time domain information by an inverse discrete fourier transform (IDFT).
FIG. 1 shows a frequency spectrum of a conventional ADSL system over plain old telephone service (POTS). Voice transmission is located at the lower 4 kHz band. From 4 kHz to 1.1 MHz, 256 channels of 4.3125 KHz each are provided. The first 5 channels (from 4–26 kHz) are usually not used for data transmission. For frequency division multiplexed systems (FDM), transmission of data is limited to channels 7 to 256. The transmission band is separated into first and second portions 140 and 160, which are use for upstream (from the end user) and downstream (to the end user) communication. The first portion ranges from 25.875 kHz to 138 kHz (e.g., channels 7–32) while the second portion ranges from 138 kHz to 1.1. MHz (e.g., remaining 224 channels). Typically, the subcarrier at Nyguist frequency is not used for data transmission. For ADSL systems over ISDN, transmission of data begins at 138 kHz (or 120 kHz depending on system characteristics) instead of 25.875 kHz, as shown in FIG. 2. The upstream portion 140 comprises 26 channels. However, due to the fact that data upstream transmission is shifted to a higher frequency, the downstream portion 160 comprises 192 channels instead of 224.
The data bandwidth of conventional ADSL systems over POTS is about 1.1 MHz. As data transfer requirements become more voluminous and complex, there is a demand to further increase the data bandwidth of conventional ASDL systems.